Water Hardness. Calculating Calcium.

[Dannalee Jayvia]

Homework Statement

I have been asked to determine the concentration of calcium ions, in a 100 mL sample of water.
Eriochrome black T Indicator was used. Magnesium was removed through a precipitate. The water sample changes colour to red. pH 10 buffer solution was also used.
the 3 test amounts of 0.01 mol EDTA solution were
11.07mL, 11.57mL and 10.57mL giving an average titre of 11.07mL

Homework Equations

moles(EDTA) = concentration (mol) x volume (L)

Calculate moles of Ca²⁺ present in the 100.0 mL water sample:
moles Ca²⁺ = moles EDTA

Calculate concentration (molarity) of calcium ions in the water:
molarity(Ca²⁺) = moles ÷ volume (L)

This is as far as I could figure out

The Attempt at a Solution

moles(EDTA) = concentration (mol) x volume (L)
= 0.01 x 11.07 x 10^-2 = 11.07 x 10^-4 mol

Calculate moles of Ca²⁺ present in the 100.0 mL water sample:
moles Ca²⁺ = moles EDTA = 11.07 x 10^-4 mol

Calculate concentration (molarity) of calcium ions in the water:
molarity(Ca2+) = moles ÷ volume (L) = 11.07 x 10^-4 ÷ 100.0 x 10^-3

 

[Borek]

In general you are on the right track, but your math is incorrect here:

0.01 x 11.07 x 10^-2 = 11.07 x 10^-4 mol

10^-2 is wrong (as long as it is intended to be a conversion factor between mL and L, I don't see what else you could use here).